1. Field of Invention
This invention relates generally to liquid developing material-based electrostatic latent image development, wherein a layer of liquid developing material is brought into contact with a latent image-bearing surface in the vicinity of an electric field for causing selective image-wise separation of the liquid developing material layer to produce a desired output image corresponding to the latent image. In particular, the invention relates to methods and apparatus for enhancing the capabilities of such printing systems to provide on-demand custom colors. The invention also relates to a selectively addressable toner applicator that can apply toner patches of varying size, shape, density and location on a toner support.
2. Description of Related Art
Generally, processes for electrostatographic copying and printing are initiated by selectively charging and/or discharging a charge receptive imaging member in accordance with an original input document or an imaging signal in order to generate an electrostatic latent image on the imaging member. The electrostatic latent image is subsequently developed into a visible image by a process in which charged toner particles are brought into the vicinity of the latent image and caused to migrate to image areas thereof. Typically, the developing material may include carrier granules having marking or toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically drawn away from the carrier granules and attracted to the latent image areas to create a powder toner image on the imaging member.
Alternatively, the developing material may include a liquid developing material comprising a carrier liquid having charged pigmented marking particles (or so-called toner solids) immersed therein. The charge on the marking particles is created by a soluble ionic surfactant, or so-called charge director material, dispersed and/or dissolved in the liquid carrier/marking particle composition to create an electrochemical reaction, which results in the exchange of ionic species between the marking particles and micelles formed by the charge director. In the case of traditional liquid developing material-based development processes, the liquid developing material is generally applied to the surface of the latent image bearing member, with the charged marking particles being caused to electrophoretically precipitate from the liquid developing material dispersion and be deposited upon the image areas of the latent image to form a developed liquid image.
Regardless of the type of developing material employed, the toner or marking particles of the developing material are typically uniformly charged and attracted to the latent image via electrostatic fields for forming a visible developed image corresponding to the latent image on the imaging member. The developed image is subsequently transferred, either directly or indirectly, from the imaging member to a copy substrate, such as paper or the like, to produce a "hard copy" output document. In a final step, the imaging member is cleaned to remove any residual developing material and/or charge therefrom in preparation for a subsequent image forming cycle.
The above-described electrostatographic process is well known and has been implemented in various forms to facilitate, for example, so-called light lens copying of an original document, as well as for printing of electronically generated or digitally stored images where the electrostatic latent image is formed via a modulated laser beam. Analogous processes also exist in other electrostatic printing applications such as, for example, ionographic printing and reproduction where charge is deposited in an image-wise configuration on a dielectric charge retentive surface (see, for example, U.S. Pat. Nos. 4,267,556 and 4,885,220, among numerous other patents and publications), as well as other electrostatic printing systems wherein a charge carrying medium is adapted to bear an electrostatic latent image. It will be understood that the instant invention applies to all various types of electrostatic printing systems and is not intended to be limited by the manner in which the image is formed on the imaging member or the nature of the latent image bearing member itself.
As described above, the typical electrostatographic printing process includes a development step whereby developing material including toner or marking particles is physically transported into the vicinity of a latent image bearing imaging member, with the toner or marking particles being caused to migrate via electrical attraction of toner or marking particles to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration. The development process is most effectively accomplished when the particles carry electrical charges opposite in polarity to the latent image charges, with the amount of toner or marking particles attracted to the latent image being proportional to the electrical field associated with the image areas. Some electrostatic imaging systems operate in a manner wherein the latent image includes charged image areas for attracting developer material (so-called charged area development (CAD), or "write white" systems), while other printing processes operate in a manner such that discharged areas attract developing material (so-called discharged area development (DAD), or "write black" systems).
Numerous and various alternative methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. Of particular interest with respect to the present invention is the concept of forming on a surface a thin layer of liquid developing material having a high concentration of charged marking particles, with the layer being acted upon by image-wise forces and separated into image and background portions. For the purposes of the present description, the concept for latent image development via direct surface-to-surface transfer of a toner layer via image-wise forces will be identified generally as Contact Electrostatic Printing (CEP). As one variant of CEP, a thin layer of liquid developer is brought into contact with an electrostatic latent image on another surface, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields. Some of the hallmarks of a CEP system include:
(a) a liquid toner medium that has a relatively high solid content (approximately 10-50%) in a form of a thin layer (approximately 5-30 microns); and PA1 (b) limited relative movement between toner particles during and after latent image development, i.e. the relatively high solid content of the toner prevents toner particles from moving relative to each other, unlike that in the traditional liquid developing mediums, and the liquid carrier in the toner serves to bind the toner particles together as a single mass, unlike the individual toner particles in powder developing systems.
Patents which may describe certain general aspects of CEP, as well as specific apparatus therefor, may be found in U.S. Pat. Nos. 5,436,706 and 5,596,396, issued to Landa et al., U.S. Pat. No. 5,619,313, issued to Domoto et al., as well as other patents cited therein.
On-demand custom color capability of electrostatographic printing systems, including CEP, may vary significantly due to numerous conditions affecting image development, among various factors including but certainly not limited to, the methods and apparatus used to mix the primary colors to achieve the desired custom color and the process controls implemented on the color mixing and development subsystems to maintain the color accuracy and stability. In general, a number of primary color developers are mixed in a reservoir with certain proportions according to the customer selection and then the developer mixture is applied to the latent image for development. Example patents which may describe certain general aspects for achieving customer selectable colors, as well as specific apparatus therefor, may be U.S. Pat. No. 5,781,828, issued to Caruthers, Jr. et al., as well as other patents cited therein.